Rollup merge of #115958 - RalfJung:mystery-plus, r=Mark-Simulacrum,notriddle

explain mysterious addition in float minimum/maximum

Thanks to `@programmerjake` for mentioning this.

library/core/src/num/f32.rs

@@ -957,6 +957,7 @@ impl f32 {
         } else if self == other {
             if self.is_sign_negative() && other.is_sign_positive() { self } else { other }
         } else {
+            // At least one input is NaN. Use `+` to perform NaN propagation and quieting.
             self + other
         }
     }

library/core/src/num/f64.rs

@@ -968,6 +968,7 @@ impl f64 {
         } else if self == other {
             if self.is_sign_negative() && other.is_sign_positive() { self } else { other }
         } else {
+            // At least one input is NaN. Use `+` to perform NaN propagation and quieting.
             self + other
         }
     }